Kassel is one of Germany's oldest cities, founded by the Celts in the 4th century BC. It's located in the middle of the country, in the state of Hesse. The city is well known for its beautiful parks, the largest of which is the Bergpark Wilhelmshöhe, a UNESCO World Heritage Site. The park is home to the Herkules, a monumental statue of the Greek god Heracles. Other points of interest in the city include the Schloss Wilhelmshöhe, the documenta art exhibition, and the Karlsaue park. Kassel is also a major transportation hub, with a large train station and an international airport.